Lenovo — auto shutdown

I have updated my lenovo k8 note in android 8.0.0 (Oreo) after updating my mobile is restarting automatically and creating problem for me.
After this update, i contacted customer care they suggested me following solutions.
1. Resetting phone
2. Stopping moto app launcher and relaunching it.
3. Resetting app preferences
4. Updating my oreo to latest version of september 5, 2018.
5. Running mobile into safe mode.
None of the solutions worked for me.
Now yesterday night (Date:01-jan-2019) it's been completely of no use. Getting restarted continuously.
When i visited below authorized service center
Service center name:today telecom
Service center address:opposite bethany school, waltair road visakhapatnam

The service center executives asking for money to update software of mobile. Even after that also it's not working now they are asking me to change mother board. When i asked about the price they are saying in the cost of that mother board you can buy new mobile.

I was trying to explain that this has happened because of the latest update provided by lenovo only why should i pay for it, so they giving me example of iphone (The battery issue happened earlier because of there latest update). What a pathetic explanation they are provided i don't believe it.
Lenovo please save yourself for getting so much famous with this issues.

Please give me some solution.